Processing of Parts

The electroless alloy coatings discussed in this article are produced from aqueous solutions following a specific base-metal-dependent cleaning and activation procedure. These procedures are generally similar to those employed for the electroless deposition of unalloyed nickel; see the article "Electroless Nickel Plating" in this Volume for more information.

Process Control. Nonelectrolytic alloy processes require more frequent sampling and analysis than conventional electrolytic plating processes. This is caused by metal salt consumption without the advantage of an anode to replace the metal, oxidation of reducing agents, lower metal concentrations, and the consumption of the alloying agent at a rate different from that of the primary metal.

The process control requires that the operator know the rate of consumption of all ingredients. After initial adjustment to optimum concentrations, the operator must replenish each of the ingredients at their individual rate of consumption.

Two types of control tests determine this replenishment on-line and off-line. On-line tests are performed during the process by analyzing for trace metals, alloying elements, reducing agents, and organics. By selecting specific organic chemistries that produce consistent alloys, maintaining these chemicals in balance, and controlling the conditions of the process, a consistent alloy can be produced. Off-line testing is performed on the alloy specimens after the plating process and is used to verify that the process control is valid.

By combined use of these control systems and scheduled replenishment during the plating process, uniform deposit properties can be achieved.

Deposit Analysis. There are several tests which can be performed on the alloy to determine its quality. They should be selected to measure a specific property in the application.

Composition is generally measured by inductively coupled plasma or atomic absorption for the major constituents. Alloys of dispersed particles require more complex procedures to ensure that the alloy as well as the percentage weight of the included particle is within specification.

Wear resistance is sometimes used to measure the quality of the deposit. The Taber abraser has been used to test abrasion resistance on dispersed hard-particle alloys. Other methods include the Falex pin-on-vee-block and LFW1 block-on-ring apparatus for testing adhesive wear. Other test methods for fretting and erosion wear are sometimes used. An important consideration in evaluating the wear rate is the number of samples required to produce statistically accurate results. Generally the most relevant test is in the field, where the parts are subjected to the actual wear system and the wear rate or service life is the measure of performance.

Uniformity of dispersed particles within the alloy may be measured to ensure consistent wear resistance. This is generally measured by examination of a metallographic cross section of a specimen processed with the parts.

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